My sunflower grown this past summer at its peak. The bloom was over 40cm in diameter. It was facing down like a shower head so I used our large home entrance mirror (my wife was not home) to shine sunlight from the ground upward. I then photographed it while comfortably laying on the lawn. The neighbors think I'm crazy of course - confirming suspicions. This allowed the flower to be bright and the sky to be very dark in contrast.